Image intensifier tubes are well known devices for enhancing night-time vision. The image intensifier multiplies the amount of incident light received from an object and produces an intensified image which can be more easily viewed. Thus, at night-time or under otherwise low light conditions, the ability to view objects is enhanced. Accordingly, devices that employ image intensifier tubes are used in a wide variety of industrial and military applications. The U.S. Military, for example, uses image intensifiers during night-time operations wherein night radiation is reflected from targets that would not otherwise be visible, and the reflected energy is amplified by the image intensifier. As a result, the target becomes visible without the necessity of additional light. Other examples of the use of image intensifiers include providing night vision to sufferers of retinitis pigmentosa (night blindness); enhancing night vision of aviators; and improved photography of astronomical objects.
Image intensifiers are well known in the industry by names for particular designs based on the generic generation from which those designs evolved. Accordingly, image intensifier tubes are typically identified by their generation number, such as the Generation 0 tube to the contemporary Generation III (GEN. III) tube. Modem GEN. III image intensifier tubes typically employ three major components--a GaAs photo-cathode, a phosphor screen (anode) and a micro-channel-plate electron amplifier. The photo-cathode converts incident light energy patterns into electron patterns which are multiplied by the electron amplifier. The phosphor screen then converts the amplified electron pattern back into a light pattern. All three components are disposed within an evacuated housing which permit electrons to flow from the photo-cathode across the electron amplifier to the phosphor screen. Examples of such devices are disclosed in U.S. Pat. No. 5,029,963 entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER, issued to Naselli et al. and assigned to ITT Corporation, the assignee herein. (This reference discusses both GEN II and GEN III image intensifier tubes).
During the fabrication of a GEN. III image intensifier tube, the photo-cathode must be cleaned and a layer of negative electron affinity material, such as cesium oxide, applied to it, prior to assembly of the photo-cathode to the vacuum tube housing. The GEN. III photo-cathode has a GaAs surface having contaminants, such as metal oxide deposits, disposed thereon which must be removed during the cleaning process. Other compound semiconductor photo-cathodes have similar contaminants which require removal.
Conventionally, the photo-cathode is cleaned using a high temperature cleaning process. The high temperature is sufficient to vaporize the surface contaminants thereby exposing a clean surface. One problem with this high temperature cleaning is that the temperature of the photo-cathode must be maintained within a relatively narrow window during the cleaning process. A typical window would be, for example, 610.degree.-630.degree. C. Due to the limitations of temperature controlling methods, it is difficult to achieve cleaning operation within this window with 100% accuracy. Temperatures below the window yield insufficiently clean cathodes, resulting in non-uniform photo-response defects. Temperatures above the window produce excess thermal strain which results in crystal slip defects.
Another problem is that the high temperature causes the dopants, such as zinc, to become depleted from the surface resulting in reduced photo-response. The high temperature also precludes the use of many face-plate glasses which are less well matched to GaAs, such as those used in fiber-optics, which cause enough thermal stress to result in crystal slip defects below the temperature required for cleaning. In addition, the high temperature precludes the use of improved cathode structures, such as thinner window layers, which cannot tolerate the thermal stress resulting from conventional process temperatures.
Moreover, to maintain the temperature within the high temperature window necessitates the use of sophisticated temperature monitoring and control equipment. Typically, the photo-cathode temperature is monitored during the heat cleaning step by employing an infrared camera which is directed towards the photo-cathode being cleaned. The radiant energy of the photo-cathode is monitored and converted to temperature data. This data is relayed to temperature control equipment which controls the heat source supplying the high temperature heat. Thus, it would be desirable to eliminate the need for complex temperature sensing and control equipment by using a low temperature process.
FIG. 1 illustrates a prior art high temperature photo-cathode cleaning apparatus 10. Photo-cathodes 15 are first loaded onto a stand 12 and transferred to receptacles 14 by means of an automatic transfer mechanism 18. Photo-cathodes 15 are then transferred to a heat cleaning stand 17 within cleaning chamber 11 by another automatic transfer mechanism 16. A high temperature heating element 13 in proximity to the photo-cathode being cleaned is coupled to control equipment (not shown) to provide the precise high temperature required. An infrared camera 19 monitors the photo-cathode temperature. When the precise temperature window is reached, the control equipment, which is also coupled to the camera 19, controls the heating element 13 to maintain temperature within the critical window. After the heat cleaning, the photo-cathode is transferred to process well 60 where a negative electron affinity layer, such as cesium oxide, is applied.
A similar apparatus to that of FIG. 1 is disclosed as a portion of an overall automated system in a co-pending U.S. Patent application Ser. No. 08/073,746 entitled AUTOMATED SYSTEM AND METHOD FOR ASSEMBLING IMAGE INTENSIFIER TUBES, filed on Jun. 8, 1993 for T. Murray, and assigned to ITT, the assignee herein.
A serious problem with non-uniform photo-response and crystal slip defects is that they are not detectable until the completed image intensifier tube can be powered on. When the tube fails at this point as a result of the defect, considerable costs have incurred.
It is therefore an object of the present invention to provide a low temperature cleaning process for photo-cathodes used in image intensifier tubes that overcome the problems of the prior art.
It is an additional object of the present invention to provide a low temperature cleaning process that does not require precise control of the cleaning process temperature.
It is a further object of the present invention to provide a low temperature conditioning process which removes the photo-cathode contaminants more reliably, thus avoiding non-uniform photo-response.
It is another object of the present invention to provide a low temperature conditioning which results in less thermal stress, and therefore avoiding crystal slip defects.
It is an additional object of the present invention to provide a low temperature conditioning which results in less temperature induced diffusion of dopants, and therefore avoiding the resulting degradation of photo-response.
It is a further object of the present invention to provide a low temperature conditioning process which will tolerate the use of face-plate materials which are less well matched to the thermal expansion properties of the active cathode material than the conventional face-plate glasses.
It is another object of the present invention to provide a low temperature conditioning process which will produce less thermal stress and accommodate cathode structures which are less tolerant of stress, such as thinner-window-layer cathode structures.
It is a further object of the present invention to provide such a low temperature photo-cathode cleaning process that utilizes the action of atomic and molecular particles, such as those produced in a plasma.
It is an additional object of the present invention to combine the action of the atomic and molecular particles with a low temperature degas to clean Third Generation and similar photo-cathodes.